U.S. Pat. No. 4,059,454 describes transparent glass articles having compositions in the Li.sub.2 O-NiO-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 or Na.sub.2 O-NiO-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 field which, when subjected to a particular heat treatment, will spontaneously exhibit the growth in situ of nickel spinel (NiO.Al.sub.2 O.sub.3) crystallites throughout the glass body to an amount not exceeding about 5% by volume. It is postulated in that application that NiO inhibits the customary action of TiO.sub.2 as a nucleating agent for silicate crystal phases. Instead, a unique interaction takes place between TiO.sub.2 and NiO to preferentially cause the production of nickel spinel crystallites which impart a greenish hue to the glass articles.
U.S. Pat. No. 4,083,709 discloses the formation of glass-ceramic articles from glasses having compositions in the Li.sub.2 O-Al.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 field that additionally contain NiO and/or CoO. Such glasses, when heat treated in an oxidizing environment at about 900.degree.-1300.degree. C. in a prescribed manner, crystallize in situ via the simultaneous growth of beta-spodumene solid solution crystals in the interior portion thereof and the growth of NiFe.sub.2 O.sub.4 and/or CoFe.sub.2 O.sub.4 crystallites in an integral surface layer thereon. The interior portions of the articles are highly crystalline and the ferrite-containing surface layers do not exceed several thousand angstroms in thickness.
U.S. Pat. No. 3,962,514 discloses the production of glass-ceramic articles having interior portions containing beta-quartz and/or beta-spodumene solid solution as the predominant crystal phase with integral surface layers containing transition metal spinel-type crystals. To illustrate such spinel-type crystals, the patent lists, inter alia, NiAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, MnFe.sub.2 O.sub.4, and CoTiO.sub.3.
The precursor glasses for such articles are noted as being in the Al.sub.2 O.sub.3 -SiO.sub.2 and Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 base fields with Li.sub.2 O or MnO.sub.2 constituting the primary modifying oxide. The glasses are nucleated with TiO.sub.2 and/or ZrO.sub.2 and one or more of the transition metal oxides is included to supply the spinel crystals.
The parent glass articles are body crystallized in situ via heat treating between about 800.degree.-1200.degree. C. Thereafter, the desired integral surface layer containing spinel-type crystals is developed in situ by subjecting the glass-ceramic article to temperatures between about 500.degree.-1000.degree. C. in a reducing atmosphere.
The products of both U.S. Pat. No. 4,083,709 and U.S. Pat. No. 3,962,514 are asserted to display magnetic or ferrimagnetic properties, but each requires a relatively high temperature heat treatment step to convert the precursor glass body into a glass-ceramic article. Moreover, the process of U.S. Pat. No. 3,962,514 requires a subsequent heat treatment of the glass-ceramic article to effect the growth of spinel-type crystals as an integral surface layer. Finally, since the products of those disclosures are glass-ceramics, the physical characteristics thereof more closely approximate those of the crystal phases developed in the interior portions thereof than those of the parent glass.